Thermal spraying, also known as flame spraying, involves the melting or at least heat softening of a heat fusible material such as metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface where they are quenched and bonded thereto. A thermal spray gun is used for the purpose of both heating and propelling the particles. In one type of thermal spray gun, the heat fusible material is supplied to the gun in powder form. Such powders are typically comprised of small particles, e.g., between 100 mesh U.S. Standard screen size (149 microns) and about 2 microns. Heat for powder spraying is generally from a combustion flame or an arc-generated plasma flame. The carrier gas, which entrains and transports the powder, may be one of the combustion gases or an inert gas such as nitrogen, or it may simply be compressed air.
Quality coatings of certain thermal spray materials have been produced by spraying at high velocity. Plasma spraying has proven successful with high velocity in many respects but it can suffer from non-uniform heating and/or poor particle entrainment which must be effected by feeding powder laterally into the high velocity plasma stream. U.S. Pat. Nos. 2,714,563 and 2,964,420 (both Poorman et al) disclose a detonation gun for blasting powdered material in a series of detonations to produce coatings such as metal bonded carbides. High density and tenacity of coatings are achieved by high impact of the powder particles, and the short dwell time in the heating zone minimizes oxidation at the high spray temperatures.
A rocket type of powder spray gun can produce excellent coatings of metals and metal bonded carbides, particularly tungsten carbide, and is typified in U.S. Pat. Nos. 3,741,792 (Peck et al.) and 4,416,421 (Browning). This type of gun has an internal combustion chamber with a high pressure combustion effluent directed through a nozzle chamber. Powder is fed laterally into the flame or into the nozzle chamber to be heated and propelled by the combustion effluent.
Short-nozzle spray devices are disclosed for high velocity spraying in French Patent No. 1,041,056 and U.S. Pat. No. 2,317,173 (Bleakley). Powder is fed axially into a melting chamber within an annular flow of combustion gas. An annular air flow is injected coaxially outside of the combustion gas flow, along the wall of the chamber. The spray stream with the heated powder issues from the open end of the combustion chamber.
Since thermal spraying involves melting or at least surface heat softening the spray material, non-meltable powders such as certain carbides and nitrides cannot be sprayed into successful coatings without incorporating a binder into the material. For example, powders may be formed by cladding a metal onto a core of non-meltable material as disclosed in U.S. Pat. No. 3,254,970 (Dittrich et al.) or vice versa as disclosed in U.S. Pat. No. 3,655,425 (Longo and Patel). However, such compositioning has not been fully sufficient for producing high quality coatings and optimum deposit efficiency with conventional thermal spray guns, vis. plasma or low velocity combustion.
Thermoplastic polymer powders such as polyethylene melt easily and many can readily be thermal sprayed. However, thermoset polymer powders generally do not melt, at least without first decomposing and/or oxidizing at the high thermal spraying temperature. Certain of these thermoset powders as disclosed in U.S. Pat. No. 3,723,165 (Longo and Durman) (assigned to the predecessor in interest of the present assignee) may undergo a superficial chemical or physical modification of the polymer surface of each particle so as to become surface heat softenable. An example is the poly (paraoxybenzoyl) ester powder described in U.S. Pat. No. 3,784,405 (Economy et al). As further explained in Example 1 of the aforementioned U.S. Pat. No. 3,723,165 such polyester may be utilized in a blend with aluminum alloy powder. Plasma spraying such a blend has been highly successful for producing abradable coatings for gas turbine engine seals and the like. However, the basic unmeltability of the polymer still results in poor deposit efficiency, so that even with the high heat available from a plasma gun, a significant portion of the polymer constituent is lost. Since this polymer is quite expensive, there is a need to improve the thermal spraying of the polymer-aluminum blend. There also has been an on-going need for improvements in abradability and erosion resistance of the coatings.
Therefore, objects of the present invention are to provide an improved method for thermal spraying non-meltable materials, to provide a method for high velocity thermal spraying particles having a non-meltable component and a heat softenable component, to provide an improved method of including non-meltable particles in thermal sprayed coatings at reasonable cost, to provide a method for thermal spraying improved coatings of certain nonmeltable carbides and nitrides, and to provide a method for producing improved coatings of certain thermoset plastics.